Protective means for sound reproducers



K. B. AUSTIN PROTECTIVE MEANS FOR SOUND REPRODUCERS Filed Sept. 26, 1928 Inventor. Kirby B. Austm by m His Attorney July 14, 1931.

Patented July 14, 1931 PAT "FICE I KIRZBY 28. AUSTIN, or SCOTIA, NEW YORK, ASSIGNOR To GENERAL ELECTRIC GOM- PANY, A coaroRATroN or NEW YORK I ritornorivn MEANS FOR souNn REPRODUCERS Application filed September 26, 1928. Serial No. 308,478.

The present invention relates to a protective means for sound reproducers of the dynamic type. Such reproducers, as is well known, are provided with an electro-mag-' netic field coil which is excited from a suitable source of direct current and with an actuating coil arranged to move in the field of the field coil, and are particularly adapted for reproducing sounds of great in volume.

It has been found in some cases that when the field coil of such a reproducer is energized or deenergize'd with the operating or actuating coil connected with the output circuit of an energized electric amplifier, an undesirable repeated pistol-like report comes from the rep'roducer until the circuits connected therewith establish a normal operating condition. This eflect on the repro'ducer is rather severe and may often result in injury to or dislocation of the movable sound reproducing element therein, such as the cone, in the cone-type of reproducer.

A further source of annoyance is the distortion caused by overloading the amplifier employed in connection withthe sound reproducer. Like the noise occasioned by energizing and deenergizing the reproducer, it detracts from a full enjoyment of the signals being reproduced, and in the case of a large auditorium size reproducer, is very objectionable.

It is the object of the present invention to provide an improved protective means for a sound reproducer which will prevent damage thereto and will obviate the objectionable noises which tend to occur when energizing and deenergizing the same in connection with an electrical amplifying apparatus. A further feature of the invention is the provision of means which permits visual control of the distortion produced by overloading the amplifying apparatus and reproducer.

My invention will be better understood from the following description, when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

The drawing is a wiring diagram of an electrical sound amplifier which is interconnected with a sound reproducer of the dynamic type and is provided with protective means embodying the invention.

Referring to the drawing, 5, 6 are amplifier tubes connected in cascade with two power or output tubes 7 and 8, in an electrical sound amplifier having input terminals 9 and output terminals 10. Terminals 10 are connected with the actuating coil 11 of a dynamic sound reproducer 12,

continued through the amplifier as a common bus wire 18, to which is connected the cathodes of the power or output tubes 7 and 8 through a grid bias resistor 19. This is a usual selfbiasing arrangement for amplifier tubes, as is well known, the output anode current of the power tubes returning to the cathodes through this resistor.

Between the grid bias resistor 19, and the negative bus 18 orthe negative supply lead for the amplifier in what may be termed the cathode return lead, is connected a resistor 20 shunted by a spark gap 21 and by a current-responsive device, such as a voltmeter 22, the last-namedconnection being made by extension leads 23. The voltmeter 22 may be located at any suitable operating point. It is preferably mounted adjacent to or in the same location as a suitable volume control means for the amplifier or sound reproducer.

In the present example, the volume control means for the sound reproducer and amplifier is indicated at 24. It is shown as a potentiometer connected with the input terminals 9 of the amplifier to control the input energy received at its terminals 25, although it may be of any other suitable type and may be located at any other suitable point in the circuit.

The resistor 20 is located in the anode circuit of the output electric discharge device of the amplifier, being in the present example in the outputcircuit of two of such devices at a suitable point in the cathode return lead, and is of such value that, with normal output anode current flowing therein, it will cause the voltmeter or indicating means to assume substantially a mid-scale position and will carry the major, portion of the anode current.

'As is well known, variations in the anode current of an amplifier or output tube indicates overloading and distortion, and by the use of this meter connected as shown and located adjacent the volume control means 24., an operator may observe any movement of the meter needle and control the volume to prevent distortion by operating the volume control means.

In any case and especially in the usual case where leads 23 or the distortion indicating circuit to the indicating meter 22 are Long, an open circuit in or between these leads, as by having resistor 20 burn out or open circuit would cause, a dangerously high voltage to be set up between these leads. The danger from this open circuit condition is somewhat lessened by having one end of the resistor 20 connected with the negative supply bus, which is preferably grounded, as indicated but in order to fully protect lines 23, spark gap 21 is provided in shunt therewith and is preferably of a very low voltage type, arranged to puncture a thin film of insulation and provide a continuous short circuit thereafter. Such types of arresters are well known and any suitable type may be employed for this purpose.

WVith the above described protective arrangement for controlling distortion, it will be seen that the indicating instrument, which may be any suitable current-responsive de vice adapted to operate in shunt with resistor 20, may be placed in any suitable loca tion in connection with any suitable control means for varying the volume of signal applied to the amplifier and reproducer, while at the same time the leads to the instrument carry only a very light current for operating the meters and are protected from becoming a dangerous source of high voltage from an open circuit occurring in the indicating resistor 20.

In the present example, the field coil of the reproducer, which is indicated at 26, is supplied in the usual manner from the same source as the amplifying apparatus; that is, from the recti ier 15, through field terminals 27 and 28 provided on the amplifier. Vith this arrangement, the power trips of the reproducer and the field of the reproducer build up to normal operation with the rectifier or supply unit when the latter is energized.

Terminal 28 is connected with the positive side or lead 16 of the rectifier 15, through a resistor 29 and a lead 30, the resistor being provided to limit the field current to a desired value. Terminal 27 is connected with the negative side or lead 17 of the rectifier through the negative bus wire 18, a portion 31 of a current-limiting resistor 32, a lead 33 and a coil 34 of a relay 35. Relay coil 34 is thus placed in series with the field winding 26 and is energized while the current is flowing through the field winding. The current fiowing through the relay coil is adjusted to' a desired value by passing a certain amount of the field winding current through a suitable shunt resistor, such as the remaining portion of the resistor 32. This is connected in shunt with coil 34 by the addition of a lead 36 between the resistor and the coil.

The relay actuates a contact-making arm 37 which is normally held by a spring 38 against a contact 39 and which when the relay is energized, closes against a contact 40. Arm 37 and contact 40 are connected in series with the actuating coil of the sound reproducer, in the present example being connected in the output circuit of the amplifier between the amplifier output transformer ell and the output terminals 10.

With this arrangement the actuating coil of the reproducer is connected with the output of the amplifier only after the field current is built up toa value suflicient to energize relay coil 34:, at which time the amplifier is in normal operation. Thus no pistol-like report of fluctuation of the moving element of the reproducer occurs in putting the apparatus into operation. Furthermore should the field winding of the reproducer or any part of the circuit connected therewith open accidentally or by actual disconnection of its leads from terminals 27 and 28, the relay coil, which is in series therewith, would instantly become dcenergized and cause the output circuit from the amplifier to be opened, thereby cutting off the actuating coil of the sound reproducer and preventing objectionable audible sounds.

To compensate for the loss of the field load under such circumstances and to prevent an open circuit, with attendant rise in voltage across the open circuit, contact 39 is connected with lead 30 through a resistor 42, while arm 37 is connected with lead 33. This arrangement places resistor 42 across or in shunt with the field and relay coil cirouit when relay coil 3a is deenergized.

By connecting arm 37 with both the output circuit of the amplifier and the low side of the field circuit, that is, lead 33, a single switching arm is sufficient to control both circuits. Any other suitable switching arrangement may however be provided for the same purpose. The relay is preferably provided with a simple single-pole doublethrow switching means having a common connection with both the relay coil on its low voltage side and with theoutput circuit of the amplifier as shown. This arrangement, in addition, provides a ground connection for the output circuit to drain static potential therefrom.

As will be readily understood from the foregoing description, my invention has the following advantages:

1. The field circuit of the reproducer is protected against the building up of dangerously high voltage across the ends of an open circuit therein.

2. The cone or movable element of the reproducer is protected against destructive surges when the amplifier is energized.

3. Objectionable noises from the reproducer when the amplifier is energized are avoided.

l. Distortions in or overloading of the amplifier may be observed and corrected at a remote point by means which does not interfere with normal operation of the apparatus.

5. The circuit to the distortion indicating means is protected against high voltage in case of open circuit therein. 7

What I claim as newand desire to secure by Letters Patent of the United States, is:

1. The combination a with an electrical sound amplifying apparatus embodying electric discharge devices, a sound reproducer having a field circuit and an actuating circuit, and a common source of energy for said apparatus and reproducer, of a protective means therefor comprising a resistor, a relay coil in series with said field circuit, and a switch actuated by said coil to one position for deenergizing said actuating circuit and connecting said resistor in shunt with the field circuit and to another position for closing said actuating circuit and interrupting the connection of said resistor with said field circuit.

2. The combination with an electrical sound reproducer having an actuating winding and a field winding and means for supplying said windings with energy for the reproduction of sounds, of a protective relay connected in circuit with the field winding and operable when deenergized to close a circuit in shunt to said field winding and to disconnect the actuating winding and operable when energized to interrupt the shunt circuit and complete the circuit of the actuating winding, thereby preventing disruptive movement of the reproducer mechanism when energizing or de- 

